This invention relates to techniques for compensating certain distortions which are experienced during the processing of information-bearing electrical signals. More particularly, the invention relates to the compensation of distortions which are experienced during the playback of information stored in binary form on magnetic recording media.
During such playback there is conventionally produced a signal whose waveform can be thought of as being the result of the superposition of two separate signals, one of which experiences a peak of one polarity (e.g. positive) when the magnetization of the recording medium reverses in one direction, and another which experiences a peak of the opposite polarity (i.e. negative) when the magnetization reverses in the opposite direction.
The informational coding of these recordings is typically such that the resulting positive and negative going signal peaks are not uniformly spaced (i.e. 180.degree. out of phase). For example, a magnetization reversal in one direction, leading to a negative signal component peak, may be followed by an opposite reversal, leading to a positive signal peak after 200 ns. However, the next following magnetization may not occur for 400 ns. Consequently, the next following negative signal peak would not occur until 400 ns after the positive signal peak, and so forth. It can readily be shown that this results in a displacement, or "pulling" of the peaks of the composite electrical signal away from the peak positions of the separate components. Since it is the timing of these peaks in the composite electrical signal which determines the ultimate decoding and recovery of the recorded information, this pulling interferes with faithful reproduction.
This problem becomes increasingly serious as the information stored on the magnetic medium is more and more densely packed because the successive signal peaks then follow each other more and more rapidly. As a result, the amount of pulling of these peaks away from their precise reversal-representing positions increases thereby causing an increasingly adverse effect on the faithfulness of signal reproduction. Indeed, when sophisticated coding is used to achieve dense information packing, even a small amount of such pulling (sometimes referred to as intersymbol interference) can create serious distortions in reproduction, including loss of whole data bits, or groups of bits, or even loss of synchronization, all of which is particularly serious when computer-type operations are to be performed by means of the recovered information.
The problems noted above are by no means new, but have been well recognized in the art, and various solutions have been proposed. Some of these proposed solutions operate upon the information as it is being recorded, and seek to predistort it in a manner which would yield an undistorted, recovered signal despite the intersymbol interference encountered during reproduction. Others of these proposed solutions (sometimes referred to as playback equalization) operate on the signal derived during reproduction.
Representative prior discussions of signal recovery, and of the problems and proposed solutions for the type of distortions under consideration are found, for example, in the following:
Increased Magnetic Recording Read-Back Resolution by Means of a Linear Passive Network PA1 Signal Equalization in Digital Magnetic Recording by George V. Jacoby PA1 IEEE Transactions on Magnetics, September 1968 U.S. Pat. No. 2,657,276, issued Oct. 27, 1953
by H. M. Sierra PA2 IBM Journal, January 1963
Such prior proposals for dealing with these problems, while successful to some degree, also left something to be desired.
Some were inordinately demanding in terms of equipment complexity. Others, restricted the freedom of choice of the reproducing equipment. Still others, due to the fact that they operated at an analog point in the signal processing, were unduly sensitive to noise, and so forth.
For example, one way in which such intersymbol interference can be reduced is to utilize high speed logic, which logic is faster than the data that is to be handled by that logic. However, this requires the data rate, and the memory technology, to lag behind that state of the art of the logic used. Such a solution is unacceptable.
Another solution which has been suggested is to narrow the pulses which are used in conjunction with the data recording system, to reduce intersymbol interference between closely adjacent data pulses. In order to accomplish this, the data is processed through a high-pass filter, thereby narrowing the resulting data pulses. Such circuits have proved to either be too complex to realistically realize, or to produce a ringing effect which can actually contribute to the intersymbol interference between adjacent data pulses.
None of the systems proposed to date have produced a realizable data processing system which is capable of removing such intersymbol interference from a series of high speed data pulses.
Accordingly, it is an object of this invention to provide an improved technique for correcting the type of signal distortion described above.
It is another object to provide such a technique which overcomes one or more of the drawbacks of previously known corrective techniques.
It is still another object to provide such a technique which is comparatively simple and inexpensive.
It is still another object to provide such a technique which is capable of application to a distorted signal which is digital in its characteristics, and therefore less subject to noise than if it were analog.